We present new deep sub-arcsecond BVI and H\alpha
observations of the edge-on Sb galaxy NGC 891 taken with the
WIYN 3.5-m telescope. These observations are on more than 10
times deeper than our earlier imaging work of this galaxy
(Howk & Savage, 1997, AJ, 114, 2463) and show an extensive
web of inhomogeneously distributed (or clumped) dust
structures far from the midplane. These dust structures are
traceable to heights in excess of 2.0 kpc, although the
number and apparent extinction of the features decreases
markedly at heights above z=1.8 kpc. The hydrogen column
densities of the dust structures are estimated to be of
order 1021 cm-2, while the masses are likely in
excess of 105 solar masses.

Our deep, high-resolution (0.9 arcsec) H\alpha images
recover the high-z diffuse ionized gas (DIG) studied in
early works. Though the DIG contains many filaments, the
distribution of DIG is much smoother than that of the
material traced by the dust absorption. In fact, there is
little direct physical association of the DIG emission and
the dusty filaments seen in absorption, though some of the
structure seen in the DIG seems to be caused by absorption
of the H\alpha emission by the high-z dusty filaments
visible in our broadband images. We suggest that the
high-z ISM in this galaxy is a multiphase medium, with the
dusty absorbing structures tracing the cold neutral medium
in the interstellar thick disk or halo of NGC 891. This
conclusion is supported by our identification of several
discrete H II regions, tracing recent star formation, at
heights between 600 and 2000 pc from the midplane. Gas in
the halo of NGC 891 evidently has a large enough pressure
(P/k > 600 K cm-3) to permit the existence of a
multiphase medium, including a dense phase (see, e.g.,
Wolfire et al. 1995, ApJ, 443, 152).

J.C.H. was supported in this work by a NASA Graduate Student
Researcher Fellowship under the grant NGT-5-50121.

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